1. Field of the Invention
The present invention relates to a method of printing a spit pattern for an inkjet printer comprising a print head having a plurality of nozzles wherein a receiving material is moved relatively to the print head and droplets of marking material are ejected from the nozzles onto the receiving material in order to form the spit pattern of dots of marking material on the receiving material.
2. Description of Background Art
In inkjet printing, nozzle failures may be caused by nozzle clogging, contamination of a plate, in which the nozzles are formed, events in which the nozzles are touched by the receiving material, and the like. Such nozzle failures are a serious threat to reliable ink jet printing and to print quality. Therefore, it is necessary to avoid a nozzle failure and to detect a nozzle failure as soon as possible after the moment in time of failure of the nozzle.
In a single pass print process, the print head and the receiving material are moved relative to one another in such a manner that each location on the receiving material is exposed to the nozzles of the print head only once. When the width of the print head is at least as large as the width of the receiving material, the receiving material may be moved past the print head in a uniform direction, or, conversely, the print head may be moved over the receiving material only once. When the print head does not cover the entire width of the receiving material, it is moved in a main scanning direction across the paper so as to print one or more lines, and the paper is then advanced in a sub-scanning direction, so that another swath of the image will be printed in the next pass of the print head. Such a single pass process is particularly vulnerable to nozzle failures because there are only limited possibilities to compensate nozzle failures by printing extra dots with other, still intact nozzles of the print head.
It is known that the risk of nozzle failures increases when a nozzle is inactive for a certain time, because the ink may dry-out in the nozzle. DE 10 2007 035 805 A1 proposes a multi-color ink jet printing method of the type specified in the opening paragraph, wherein the risk of nozzle failure is reduced by causing the nozzles to “spit” onto the receiving material from time to time even when the print data do not command a dot to be printed. In order to hide the extra dots from human perception as far as possible, the spit pattern is designed such that each extra dot will be superposed with a dot that is printed in another color, so that the extra dot is covered by a “regular” dot, or at least the extra dot does not significantly change the visual impression, because a dot, though in a different color, would have to be present at the dot location, anyway.
Another approach to improve reliability in ink jet printing involves an automatic nozzle failure detection, which permits to take measures for removing the nozzle failure before a larger number of defective images are printed. For example, nozzle failure may be detected by printing a test pattern and then inspecting the test pattern from time to time. However, this method implies a waste in paper and marking material, especially when the test is repeated in short intervals. Moreover, this method requires a sheet disposal trajectory in the paper pass of the printer, so that the sheets carrying the test pattern may be disposed.
Another method of nozzle failure detection involves inspecting the image that has been printed in accordance with the print data. This is advantageous, since a nozzle failure can be detected immediately, and the running print process may be stopped, if necessary. However, depending on the nature of the print data, it may be difficult to detect nozzle failures, and when a nozzle failure occurs at a nozzle which is not currently used for printing, the failure cannot be detected before the nozzle is used again.
U.S. Pat. No 7,393,077 B2 discloses a method of nozzle failure detection wherein, in a first step, only specific dots that shall be used for nozzle failure detection are printed on the receiving material, these dots are then inspected for the purpose of nozzle failure detection, and then the inspected area of the image is moved past the print head in a second pass so as to print the rest of the image in accordance with the print data. Consequently, this method requires a multi-pass print process. It is further observed in this document that the dots for nozzle failure detection do not have to form part of the image to be printed in accordance with the print data but should in any case be located in a low visibility area of the image, especially an area in which the spatial frequency of the image to be printed is within a certain range.
U.S. Application Publication No. 2010/0091053 A1 describes a spit pattern, which is included in the print data, wherein a location of a dot to be ejected according to the spit pattern is determined by means of a dither matrix of entries arranged in rows and columns, each entry comprising a natural number. The spit pattern is thus constructed independently of any user selected image to be printed, while paying careful attention to the characteristics of the human visual system. The spit pattern is constructed according to a bi-level bitmap, which is directly derived from a dither matrix by means of a threshold value. An entry of the dither matrix having a value lower than or equal to the threshold value corresponds to an entry in the bi-level bitmap having a value of one. An entry of the dither matrix having a value higher than the threshold value corresponds to an entry in the bi-level bitmap having a zero value. The dither matrix may be a white noise matrix, a random periodic matrix or a blue noise matrix. A dither matrix is normally used for printing an image, but may also be used for printing a spit pattern.